1. Field of the Invention
The present invention relates to a method and a device for non destructive evaluation of defects in a metallic object by eddy currents.
The invention more specifically relates to a method for non destructive evaluation of defects in a metallic object by eddy currents, the method comprising the steps of emitting at least one alternating electromagnetic field at at least one first frequency fi in the neighbourhood of the metallic object and detecting through at least one magnetoresistive sensor a response signal constituted by a return electromagnetic field which is re-emitted by eddy currents induced by the alternating electromagnetic field in said metallic object.
2. Description of the Related Art
The technique of detection of defects in metallic objects by observation of the deformation of eddy currents lines has been well known and widely used for a long time.
The principle is to use an electromagnetic field emitter in the neighbourhood of the metallic object for creating eddy currents in this object. These eddy currents retransmit a radiofrequency field which can be detected by a receiver. In case of a presence of a defect, the current lines are perturbed and the electromagnetic field which is reemitted is modified in its amplitude and its phase distribution but its frequency remains unchanged.
Classical approaches use inductive coils as emitter of a radiofrequency field and as receiver of the re-emitted radiofrequency field.
More recently, the use of other kinds of sensors, like magneto-resistive sensors, has been proposed for the receiver part. Magneto-resistive elements (MREs) based on the so called “giant magnetoresistive effect” (GMR effect) have been demonstrated, for example in spin valves consisting of two adjacent magnetic layers, whose resistance R varies as a function of the applied magnetic field. In these GMR devices, variations of resistance ΔRmax have been observed with ΔRmax/R-values as large as 15%.
Some patents and publications show magneto-resistive sensors applied for eddy current (EC) testing.
For instance, in the document US2005/0140355 A1, a specific embodiment of the excitation coil on a printed circuit board (PCB) is used and a magnetoresistive (MR) sensor is located on the opposite side of the PCB to detect the magnetic field related to the eddy currents modified by a defect.
In the document US2005/0062470 A1, an eddy current probe having a non planar form allows the displacement of the probe in a close proximity to the object's surface avoiding an adsorption phenomenon due to static friction.
In these documents, the designs of the excitation loop and the sensor are addressed to improve the detection.
In the document U.S. Pat. No. 6,693,425 B2, the excitation current in the coil varies to obtain a varying field penetrating in the component under test. This invention allows scanning defects of varying depths.
In all these cases, even if the sensitivity is increased, the sensor which might be either a coil or a magnetoresistive element, detects the signal of the eddy current at a frequency equal to the excitation coil's frequency, and is therefore limited in term of signal-to-noise ratio.